1. Field of the Invention
This invention relates to improvements in an airbag module including an airbag which is folded and stored inside a cover, and more particularly to the airbag module which is arranged to rapidly break or tear the cover so as to smoothly inflate and develop the airbag.
2. Description of the Prior Art
Airbag modules have been proposed and put into practical use in such a manner that one of them is installed in a boss section of a steering wheel in order to protect a driver of an automotive vehicle from impact at a vehicle collision or the like. Such an airbag module is, for example, disclosed in Japanese Patent Provisional Publication No. 6-312643 and includes an airbag whose major part is folded and located facing the driver and positioned in front of an inflator for ejecting gas. In other words, the major part of the folded airbag is located between the upper surface of the inflator and the front side of a cover for covering the airbag. The inflator, airbag and cover are secured to a base plate. With this arrangement, when a vehicle collision is detected or when a high deceleration of the vehicle over a predetermined level is detected, the inflator ejects gas to inflate the airbag thereby breaking or tearing the cover along a predetermined tear line under the pressure of expansion of the airbag. This causes the cover to open taking the shape of opened doors, so that the airbag projects through the cover and inflates and develops in front of the driver or vehicle passenger.
The airbag module of this type is usually arranged such that a horn device is switched ON by pressing the cover in order to improve the operationability of a horn switch. For example, in the arrangement of Japanese Patent Provisional Publication No. 6-312643, the whole airbag module is supported by horn plates and springs, in which the whole airbag module is moved downward by pressing the cover thereby allowing the movable contact disposed on the horn plates to come into contact with a stationary contact disposed on the side of a steering wheel main body thereby closing a switching circuit to sound the horn device.
Besides, with an arrangement disclosed in Japanese Patent Provisional Publication No. 7-40838, a diaphragm type switch is installed to the back side surface of a cover of an airbag module and covered at its back side surface with a stiff panel. When the cover is pressed by a driver, the diaphragm type switch is pressed through the cover thereby to close a switch circuit.
With the above-discussed conventional arrangements, it has been desired to rapidly break or tear the cover when gas is ejected from the inflator in order to smoothly inflate and develop the airbag. In this regard, in the arrangement of Japanese Patent Provisional Publication No. 6-312643 in which the major part of the airbag is folded and stored to be located at the side of the vehicle occupant, gas is supplied into the airbag which has been folded a plurality of times so that the inflating airbag thrusts the central section of the cover. At this time, the central section of the cover elastically deforms in a manner to bulge upwardly, and therefore it is difficult to effectively apply a breaking force to a tear line. This makes it difficult to rapidly break or tear the cover of the airbag module.
Concerning the above conventional switch arrangements in which the switch is operated by pressing the cover of the airbag module, the airbag is folded to be stored in a space having a certain volume which space is located inside the cover; however, the airbag cannot be always folded in the same condition. As a result, a hardness of the airbag in the folded state is not constant, so that a deformation amount of the airbag or a force required to deform the cover with the airbag is not constant when the cover is pressed. This makes a dispersion in depression stroke of the switch throughout many products of the airbag module thereby lowering the operationability of the switch combined with the airbag module.